Bibliographic details of the publications numerically referred to in this specification are collected at the end of the description.
Bacterial infection represents a major cause of mortality and morbidity in human and other animal populations. One important group of bacteria are the mycobacteria. The mycobacteria are defined on the basis of a distinctive staining property conferred by their lipid-rich cell walls. The mycobacteria are relatively impermeable to various basic dyes but once stained, they retain dyes with tenacity. The mycobacteria have been referred to as “acid-fast” bacteria since they resist decolorization with acidified organic solvents (1). Mycobacteria range from widespread innocuous inhabitants of soil and water to organisms responsible for devastating and chronic diseases notably in tuberculosis and leprosy caused by Mycobacterium tuberculosis and Mycobacterium leprae, respectively.
Leprosy involves infection in skin tissue and can lead to disfigurement. Tuberculosis is generally confined to internal organs. Although both leprosy and tuberculosis were largely controlled by chemical intervention and improvements in living conditions, tuberculosis is now re-emerging as a major health problem. On an annual basis, reportedly between 2 and 3 million people die from tuberculosis, mostly in developing countries (2).
Conventional diagnostic tests for tuberculosis include chest X-ray, detecting the presence of acid-fast bacilli in clinical specimens and the skin test using tuberculin PPD (Purified Protein Derivative) [3]. However, these procedures are time intensive, frequently the results are ambiguous and X-ray machines are expensive and generally not portable enough for use in developing countries.
Nucleic acid probes can be used in a polymerase chain reaction (PCR) to specifically detect a mycobacterial infection, but require complex equipment, highly skilled staff and are too expensive for the developing countries (4). Rapid serological diagnostic tests are available on an ELISA or “strip” format which uses antigen(s) to detect antibody in sera (5).
However, currently there is no satisfactory test for tuberculosis. A majority of M. tuberculosis antigens studied to date have homology with analogues proteins of other microorganisms that may or may not be pathogenic; resulting in cross-reactivity of these antigens to reactive serum antibodies in patients with inactive TB or nontuberculous infections (6). Hence, positive test results produced by these known antigens are generally unreliable and supplementary tests are required to confirm the presence of the tuberculosis infection.
In work leading up to the present invention, the inventors sought to use recombinant molecules from M. tuberculosis in the development of a highly specific and sensitive diagnostic test for tuberculosis. The same or similar molecules are also proposed for use as therapeutic agents for the treatment of tuberculosis.